Pneumatic tires have sidewalls which are conventionally desired to have good scuff resistance (abrasion resistance) and resistance to atmospheric degradation and particularly resistance to ozone degradation. A suitable visual appearance of the tire sidewall surface may also be a desirable property.
Scuff resistance for a tire sidewall rubber composition is typically promoted by inclusion of a high cis 1,4-polybutadiene rubber which is well known to those having skill in such art.
Resistance to atmospheric degradation (e.g. ozone degradation resistance) might be promoted by an inclusion of a saturated elastomer in the sidewall rubber composition, namely an elastomer having little or no olefinic unsaturation such as, for example, an ethylene/propylene/non-conjugated diene rubber (EPDM) or a brominated copolymer of isobutylene and para-methylstyrene. See, for example, U.S. Pat. No. 6,255,397.
Tire rubber sidewalls have heretofore been proposed which contain a high trans 1,4-polybutadiene polymer to promote resistance to tear and improved flex fatigue. See, for example, U.S. Pat. No. 5,626,697. It is to be appreciated that such patent relates to the use of high trans 1,4-polybutadiene polymer in a tire sidewall application without teaching use of saturated type polymers, without significant olefinic unsaturation such as, for example, brominated copolymers of isobutylene and para-methylstyrene as well as EPDM (ethylene/propylene/conjugated diene) type of polymers. The rubber composition of the patent also contained staining antidegradants to provide protection to atmospheric ozone for the diene-based polymers.
However, use of high trans 1,4-polybutadiene rubber, in blends with a brominated copolymer of isobutylene and para-methylstyrene, has been observed to provide improved tear strength (resistance to tear) and improved adhesion to ply and wire coat rubber compositions as compared to blends of high cis 1,4-polybutadiene and a brominated copolymer of isobutylene and para-methylstyrene.
In practice, it remains desirable to provide a rubber tire sidewall of high tear strength and adhesion to other tire components which also possesses suitable ozone resistance and a good black colored surface appearance in the absence of amine-containing antidegradants such as, for example, staining antioxidants or antiozonants.
For this invention, a tire sidewall rubber composition is presented which contains a combination of brominated copolymer of isobutylene and para-methylstyrene and high trans 1,4-polybutadiene polymer instead of a high cis 1,4-polybutadiene rubber in order to improve tear strength and adhesion to rubber tire components such as, for example rubber coated carcass plies and rubber coated wire reinforcement.
In the description of this invention, the term “phr” where used relates to parts by weight of an ingredient per 100 parts by weight of rubber, unless otherwise provided.
The terms “rubber” and “elastomer” are used interchangeably unless otherwise indicated. The terms “vulcanized” and “cured” are used interchangeably unless otherwise indicated. The terms “compound” and “rubber composition” may be used interchangeably unless otherwise indicated.